The present invention pertains to the communication art and, more particularly, to a means for controlling the level of signals carried over a duplexed line.
Controlling the level of signals carried over a duplexed transmission line has been a long established problem in the communication art. The problem arises as a result of insertion loss of long transmission lines. For example, if two stations are separated by twenty miles of conventional telephone line, the signal received by one station from the other may be attenuated 20db from its original level. In many applications, it is imperative for successful operation of the system that compensation be made for transmission line losses. For example, at the receiving station wherein an operator handles a headset, if the operator's signal is some 20db above that of the remote signal, feedback within the operator's headset may obliterate the other station's signals. Further, in applications wherein either the remote station or base station signals are fed to a transmitter, it is important that the level of signals applied to the transmitter be constant.
One prior art approach to compensating for transmission line losses has been the use of two independent transmission lines between the base and remote stations. Fixed gain amplifiers are provided in each line thus exactly compensating for line losses. This system, while effective, ties up two transmission lines and thus, is quite expensive.
A second approach has been the use of complicated hybrid circuitry which provides precise impedance matching and transformer coupling to the transmission line such that the independent signals carried by the line may be separated and individually amplified. Such hybrids are extremely costly to manufacture and may require periodic adjustment for optimum operation.
The prior art has developed numerous automatic gain circuits which help assure that signal levels are maintained at a desired level. Such systems have exhibited two fundamental problems. Firstly, the gain control circuit must establish the relative level of a processed signal. This invariably takes a fixed period of time, during which noise bursts or periods of very low volume might occur, dependent on the initial state of the gain control circuit. In addition, the prior art gain control circuits have worked off of the average level of the signal being processed on a continuous basis. Thus, for transmissions, such as pauses between words in a message, prior art gain control circuits sense a very low signal level thereby increasing gain and amplifying background noise. Now when the next word is processed the initial portion thereof is amplified by a high gain factor thereby resulting in an annoying burst.